Diagnostic assays are widespread and central for the diagnosis, treatment and management of many diseases. Different types of diagnostic assays have been developed over the years in order to simplify the detection of various analytes in clinical samples such as blood, serum, plasma, urine, saliva, tissue biopsies, stool, sputum, skin or throat swabs and tissue samples or processed tissue samples. These assays are frequently expected to give a fast and reliable result, while being easy to use and inexpensive to manufacture. Understandably it is difficult to meet all these requirements in one and the same assay. In practice, many assays are limited by their speed. Another important parameter is sensitivity. Recent developments in assay technology have led to increasingly more sensitive tests that allow detection of an analyte in trace quantities as well the detection of disease indicators in a sample at the earliest time possible.
A common type of disposable assay device includes a zone or area for receiving the liquid sample, a conjugate zone also known as a reagent zone, and a reaction zone also known as a detection zone. These assay devices are commonly known as lateral flow test strips. They employ a porous material, e.g., nitrocellulose, defining a path for fluid flow capable of supporting capillary flow. Examples include those shown in U.S. Pat. Nos. 5,559,041, 5,714,389, 5,120,643, and 6,228,660 all of which are incorporated herein by reference in their entireties.
The sample-addition zone frequently consists of a more porous material, capable of absorbing the sample, and, when separation of blood cells is desired, also effective to trap the red blood cells. Examples of such materials are fibrous materials, such as paper, fleece, gel or tissue, comprising e.g. cellulose, wool, glass fiber, asbestos, synthetic fibers, polymers, or mixtures of the same.
WO 2007/012975 discloses a hybrid device that includes a capillary channel having bifurcations that is stated to help present a more united fluid front to the resuspension chamber, and thereby increase the speed of detection of a substance and improve the accuracy of detected results. US 2009/0311805 discloses an assay device having deposited conjugate in a conjugate zone. U.S. Pat. No. 6,271,040 discloses an assay device having a reaction chamber 4 that includes dried or lyophilized powders. The shape of the reaction chamber is disclosed as being such that the movement of the reaction mixtures from the reaction chamber is not turbulent and eddies are not formed as a result of the movement out of the reaction chamber.
Another type of assay device is a non-porous assay having projections to induce capillary flow. Examples of such assay devices include the open lateral flow device as disclosed in WO 2003/103835, WO 2005/089082, WO 2005/118139, and WO 2006/137785, all of which are incorporated herein by reference in their entireties.
US 2009/0311805 discloses an assay device having projections to induce capillary flow. The '805 application also discloses that a substance (i.e., reagent material) can be deposited between the projections in a substrate zone and can be dissolved by sample flowing through the substrate zone. The '805 application further discloses that the shape of the zone where the substance is applied, can be adapted to control the dissolution rate and/or how the dissolved substance is distributed in the flow of liquid sample. The '805 application further provides examples of shapes that can include a triangle, a square, a rectangle, a parallelogram, a rhombus, a trapezoid, a quadrilateral, a polygon, a circle, an oval, and truncated shapes such as a half circle, a half oval, a half polygon, and a circle segment.
A known non-porous assay device is shown in FIG. 1. The assay device 1 has at least one sample addition zone 2, a reagent zone 3, at least one detection zone 4, and at least one wicking zone 5. The zones form a flow path by which sample flows from the sample addition zone to the wicking zone. Also included are capture elements, such as antibodies, in the detection zone 4, capable of binding to the analyte, optionally deposited on the device (such as by coating); and a labeled conjugate material also capable of participating in reactions that will enable determination of the concentration of the analyte, deposited on the device in the reagent zone, wherein the labeled conjugate material carries a label for detection in the detection zone. The conjugate material is dissolved as the sample flows through the reagent zone forming a conjugate plume of dissolved labeled conjugate material and sample that flows downstream to the detection zone. As the conjugate plume flows into the detection zone, the conjugated material will be captured by the capture elements such as via a complex of conjugated material and analyte (as in a “sandwich” assay) or directly (as in a “competitive” assay. Unbound dissolved conjugate material will be swept past the detection zone into the at least one wicking zone 5.
An instrument such as that disclosed in US 20060289787A1, US20070231883A1, U.S. Pat. No. 7,416,700 and U.S. Pat. No. 6,139,800 all incorporated by reference in their entireties is able to detect the bound conjugated analyte and label in the reaction zone. Common labels include fluorescent dyes that can be detected by instruments which excite the fluorescent dyes and incorporate a detector capable of detecting the fluorescent dyes. Such instruments have a read window that has a width that is typically on the order of 1 mm, which is a generally sufficient width to read enough signal, subject to an adequate width of the conjugate plume.
One drawback with such known assay devices such as those described above, is that the dissolved conjugate stream in the reaction zone is often narrower than the read window of the instrument, which may negatively impact assay sensitivity and variability. This is of particular concern for designs such as those described above where the conjugate material is deposited in the center of the conjugate zone and is dissolved from the sides as sample is flowing past. If the channel is made wider than the read window, although the dissolved reagent width may match the read window size, the fluid sample outside the read window contributes no signal and is wasted. Another drawback is that the dissolved reagent is not adequately mixed with the sample by the time it reaches the reaction zone, with the result being a lower signal in the middle of the reaction zone because dissolved reagent has local higher concentration and needs to diffuse to mix with sample further away from the reagent, and to bind with the analyte, and hence less signal being read by the read window of the instrument.
The sample size for such typical assay devices as shown in FIG. 1 are generally on the order of 200 μl. Such a sample size requires a venous blood draw from a medical professional such as a phlebotomist. There is an increasing need for lateral flow devices that are able to function with a much smaller sample size to accommodate the amount of blood available from a so-called “fingerstick” blood draw, which is on the order of 25 μl or less. Such a small amount of sample is the amount of blood in a drop of blood after pricking a fingertip with a lancet. Home blood glucose meters typically use a drop of blood obtained in such a fashion to provide glucose levels in blood. Such a smaller sample size would not require a medical professional to draw the blood and would provide greater comfort to the patients providing the sample for analysis.
To reduce the sample size required, the dimensions of the lateral flow assay devices are reduced to accommodate the smaller sample size. However, it has been found that reducing the sample size and dimensions of the device provides inadequate conjugate in the detection zone and accordingly less signal that can be read by the instrument. The inadequate conjugate in the detection zone is believed to be due to reduced sample size and inefficient use of the sample in the device, amongst other conditions. Another drawback of reducing dimensions is that the width of the detection zone will also be reduced, again making less signal available that can be read by the instrument.
Accordingly, there is a need for an assay device that can provide a wider reagent plume in the detection zone, better mix the dissolved reagent and sample, and make more efficient use of sample in an assay device, particularly in those devices where the conjugate material is deposited in the center of the conjugate zone and is dissolved from the sides.